An RFID reader—also referred to as a read/write device—of an RFID system, according to prior art, consists of an RFID antenna, which is composed of at least an antenna loop, which constitutes an inductivity and is made up of one or more coils, and of an tuning circuit. Said antenna is connected with a read/write station, which comprises a transmitter, receiver and control unit. The RFID antenna and read/write station here frequently constitute a structural unit and are referred to as an RFID reader.
The RFID antenna of an RFID system has the following tasks: first, to transmit power to the transponder and, second, to transmit data to and from the transponder. The power and data transmission is based on the magnetic coupling of the AC fields of the reader and of the transponder in the vicinity of the antennae. Here the shape of the RFID antenna has a decisive influence on the coupling to the transponder. The magnetic field strength that is effective for a transponder at a defined distance from the RFID antenna depends, among other factors, on the current that flows through the antenna and on the size of the RFID antenna.
One job of an RFID antenna is power transmission to the transponder. For this purpose the RFID antenna is itself supplied with power by a transmitter. To transmit current optimally from a power amplifier of the transmitter unit of the reader to the antenna, the power amplifier and RFID antenna must have the same input and/or output resistance (impedance). An RFID antenna therefore requires a particular input impedance so that power is transmitted optimally from the power amplifier to the antenna.
In addition, the reader antenna must be tuned as closely as possible to the operating frequency of the RFID system in order to ensure strong current and thus a high magnetic field strength.
To adjust the input or output impedance and to adapt the reader antenna to the operating frequency, a tuning circuit is used that is placed usually in the immediate proximity of the antenna.
Transponders are known in the art that consist of a microelectronic component or integrated circuit (IC) and of a resonance capacitor and an antenna coil, such that the resonance capacitor is often already integrated into the microelectronic component. The antenna coil and resonance capacitor form an electrical oscillator circuit and are tuned to their operating frequency of, for instance, 13.56 megahertz (MHz).
If a transponder comes into the RFID antenna's detection range, then the transponder receives power via the magnetic coupling with the antenna to operate the integrated switch (IC). The power level depends on the field strength and/or the number of field lines that penetrate the transponder, and the angle of the field lines to the transponder. With sufficient power, the microelectronic component is set into startup condition and starts to function.
The range in which the transponder is adequately supplied with power and can communicate with the RFID system is called the detection range.
If the vicinity of an tuned RFID antenna is modified, such as if the RFID antenna is installed close to a metallic plate, then said RFID antenna is detuned by this environment, that is, the antenna no longer operates at the previously adjusted operating point. Because of this changed environment of the RFID antenna, communication with transponders is adversely affected because the maximum possible power is no longer available in the antenna.
Multiple applications for RFID readers are possible. RFID readers are used, for example, for access control systems, ticketing systems, in libraries and logistical applications, and in electronic payment systems. Thus RFID readers are installed on the walls of houses or in automatic machines and terminals or integrated into mobile devices. RFID readers are found today, for instance, on ticket automats, drink vending machines, or turnstiles at recreational facilities, and so on. In such cases, RFID readers are often operated in the direct vicinity of metallic or otherwise electrically conductive materials. It is thus a requirement that the RFID reader should be capable of operating in different environments and of being integrated as well as possible into the particular environment. Its structure must therefore be designed to allow the simplest possible integration into other devices, such as terminals, and thus to let it function optimally, independently of surrounding structures.
One field of application is the so-called payment systems. These payment systems include the EMV® Contactless Specifications for Payment Systems, Book D, EMV Contactless Communication Protocol Specification Version 2.1, March 2011 (also referred to hereinafter as EMVco specification), which was established by current suppliers of credit cards and accordingly is to be maintained.
The EMVco specification defines an “operating volume” as an area above the RFID antenna in which a minimum field strength must be reached but a maximum admissible field strength may not be exceeded. In addition, the EMVco specification defines the signal forms generated by the RFID reader within the operating volume for communication with a transponder.
Apparatuses are known in the art that serve to tune an RFID antenna manually or automatically to its optimal working point. These apparatuses have the disadvantage, however, that they are relatively complex and are not economically usable for small, cheap RFID readers or else are complex to operate and require trained staff and expensive measuring instruments for precise adjustment.
Also known in the art are RFID readers that can be screwed onto the surface of a device, for example an automatic teller machine. To allow RFID antennae of these RFID readers also to be installed on a metallic base, it is common to install between the RFID antenna and the installation surface a ferrite layer, which shields the antennae from the metallic base. This structure of the mounted RFID reader has the disadvantage that the RFID readers cannot blend into the visual design of the automat and can be protected from vandalism only with difficulty.
The prior art includes RFID readers that correspond to the EMVco specification and comprise a housing conceived for installation onto flat surfaces. The housings of these RFID readers have, for example, a surface measuring 70 mm by 60 mm and a thickness of 18 mm or a surface of 100 mm by 70 mm and a thickness of 16 mm and meet requirements in terms of field strength and signal shapes of the EMVco specification provided they are not installed on a metallic base. By being mounted on a metallic base, however, these antennae of RFID readers known in the art are detuned and the antenna induces a great part of the generated power into the metallic base, so that neither the minimum field strength required by EMVco specification of all points of the operating volume is measurable, nor do the signal shapes correspond to EMVco specification. Even if the antenna of such an RFID reader were again tuned to the metallic base, and the antenna would again operate at its functional point, these RFID readers do not reach the required minimal field strength in this environmental situation, although the antennae of these RFID readers known in the art are at a distance of more than 10 mm from the metallic base.
An antenna of an RFID reader with an antenna surface measuring, for example, 65 mm by 55 mm requires a transmitting power of about 250 milliwatts (mW) to reach the minimum field strength at the upper border of the operating volume according to the EMVco specification if the antenna is not influenced by metal. If this antenna is brought within 10 mm of a metallic surface, the RFID reader already requires approximately 600 mW to reach the minimum field strength at the upper border of the operating volume.